I. Field of the Invention
This invention relates to Fabry-Perot modulators and in particular, but not exclusively, to those employing a multiple quantum well active region.
II. Prior Art and Other Considerations
Since the first observation of the effect of electric fields on the optical properties of GaAs-AlGaAs quantum well (QW) structures a number of optoelectronic devices have been demonstrated which exploit the enhanced electro-absorptive properties of QW structures, e.g. fast intensity modulators and hybrid optical logic elements, both bistable and non-bistable. These devices employ a QW layer or multiple quantum well (MQW) grown epitaxially as the intrinsic region of a pin diode that can operate as an electro-absorptive modulator and efficient photodetector simultaneously.
A contrast ratio (on:off) of 2:1 has been observed in transmission devices with only 1 .mu.m of MQW absorber usually consisting of wells and barriers each about 100.sup.++ thick. This is very efficient, given the device size, but a better contrast ratio is desirable. By `contrast ratio` is meant the ratio of the high:low output states, irrespective of whether the device switches on or off with applied bias. `Modulation depth` is the absolute change in state, which in reflection or transmission terms can only be between 0 and 1.
It might at first seem that in order to obtain better contrast or modulation figures one would simply increase the thickness of the MQW layer. However, the situation is complicated by a variation of the electric field across the intrinsic region of the PIN device which results from the relatively high background doping level of this layer. The background doping level has had a lower limit of 1.times.10.sup.15 /cm.sup.3 in the best available material (this is not a fundamental limit and depends on the material and growth conditions), and is routinely two or three times this value. The resultant significant variation in the electric field causes a broadening of the absorption edge of the MQW material even at zero bias and, moreover, produces a different red shift of the excitonic absorption in each well as an external bias is applied to the device for modulation. As the bias is increased the absorption edge broadening becomes worse due to the roughly parabolic dependence of the edge shift on applied field. So, instead of producing a larger change in the intensity of a transmitted or reflected beam of light the increase in thickness of the absorbing layer might only serve to distribute absorption changes over a wider spectral region and leave the modulation at the operating wavelength relatively unaffected.
Optimisation calculations have shown that if the residual doping is 2.times.10.sup.15 /cm.sup.3 it is best to use about 45 wells of 100 .ANG. GaAs separated by barriers of 100 .ANG. Al.sub.0.3 Ga.sub.0.7 As confirming the limit of around 1 .mu.m for the total thickness.
A second disadvantage of increasing the MQW thickness is that an increased voltage is required to induce a given change in absorption. It has been proposed that QW devices be integrated in 2-dimensional arrays with Si-based LSI electronics to form high-bandwidth optical interconnects, and in this case drive voltages for such modulators or logic gates will be limited to a few volts.
One method of improving modulation in a device that has limitations on its absorber thickness and drive voltage is to increase the effective optical path length by incorporating the MQW pin diode into a Fabry-Perot etalon.
Asymmetric Fabry-Perot modulators (AFPMs) containing quantum wells have recently been demonstrated, which exhibit contrasts of 13-20 dB in reflection with low insertion loss and 9-10V bias (2,3). In this case the front and back cavity mirrors are formed by the air/semiconductor interface (R0.3) and an integrated semiconductor multiple quarter-wave stack (R&gt;0.95) respectively. The enhanced modulation is achieved by using the attenuating effect of MQW electro-absorption at a resonant wavelength of the Fabry-Perot cavity in order to match front and effective back mirror reflectivities, at which point the net cavity reflection must fall to zero. Devices of this type can be readily made in planar arrays with low coupling losses, and are therefore of considerable interest as interface elements for optical interconnects.
Compatibility of the modulator voltage swing with that directly attainable in high-speed electronic circuits (5V or less) is clearly desirable. To achieve these lower drive voltages, a number of options are available. Firstly, the number of wells can be reduced. Recent calculations have indicated than an AFPM containing 31.times.(100 .ANG. GaAs well+60 .ANG. Al.sub.0.3 Ga.sub.0.7 As barrier) should achieve a peak contrast of 10 dB at only 3.3V bias (4).
Secondly, if the cavity finesse is increased by integrating a front mirror of higher reflectivity than the usual 0.30, the critical amount of absorption required for the zero off state is reduced (5). This option has recently been demonstrated by Yan et al. (6), who achieved over 7 dB contrast with only 2V bias. A disadvantage is that the increase in finesse leads to reduced optical bandwidth and higher insertion losses for a given background absorption, along with increased sensitivity to temperature and cavity thickness fluctuations.